Implantable valve prostheses may for instance be used as a one-direction-valve prosthesis in the human or animal body, especially within or nearby blood vessels. A suitable valve prosthesis needs to open readily, cause no or only a little turbulence in the blood flow, and avoid regurgitation.
An aging population and increasing welfare lead to a higher demand for cardiovascular surgeries and their related products. Also, humans tend to demand more from their body since they live longer and perform increased physical exercise. Cardiovascular products, such as heart valve prostheses, are thus subject to increased requirements with respect to loading conditions, both in magnitude as in number of cycles. Typically, heart valve leaflets may undergo over a billion load cycles in their lifetime. Durability is therefore an important requirement.
An implantable valve prosthesis is described in NL1008349. The valve prosthesis of NL1008349 comprises a support structure carrying a number of valve leaflets. The valve leaflets are made by winding reinforcing fibers onto a mandrel in specific directions, as required by the occurring stresses in the leaflets. Since the fibers have to be positioned exactly according to the maximum stress lines, the known valve prosthesis is difficult to make and uses many wound layers to accommodate stresses, whereby mass is added. Moreover the known valve prosthesis can be improved further with respect to its durability, particularly in view of the increased requirements imposed on implantable valve prostheses nowadays.
Another implantable valve prosthesis is known from U.S. Pat. No. 6,726,715. As in NL1008349, the disclosed valve prosthesis comprises leaflets that are reinforced with fibers oriented along lines of stress in the leaflets. In this way, local variations of stress are accounted for. The valve prosthesis according to U.S. Pat. No. 6,726,715 suffers from the same disadvantage as the valve prosthesis of NL1008349, among others in that it is difficult to make.
WO 2004/032987 concerns a medical device having at least three layers of polymeric components arranged in a sandwich construction of which the polymeric component of the middle layer has a shorter chain length than the other polymeric components. A heart valve is mentioned as one possible application of the sandwich construction.
The object of the present invention is therefore to provide an implantable valve prosthesis that is readily manufactured and yet shows equal or improved durability over the prior art. Another object of the present invention is to provide a method for manufacturing such an improved valve prosthesis.
This object is achieved by providing an implantable valve prosthesis, comprising at least one valve leaflet and optionally a support structure for the at least one valve leaflet, the valve prosthesis being made of a material structure of unidirectional reinforcing elements of drawn ultra high molecular weight polyolefin, extending in at least two directions, wherein the modulus of elasticity of the polyolefin reinforcing elements is at least 60 GPa. By using the described ‘two-directional’ material structure, the valve prosthesis according to the invention is easily manufactured. Moreover it exhibits improved durability. This is surprising since it is well known that a high modulus of elasticity generally is associated with more brittle materials, thus yields unfavourable durability, in particular fatigue properties, due to the higher stress levels incurred.